A particular problem associated with arrays of radiation detectors relates to controlling the lateral collection of photoexcited charge carriers, electrons or holes, such that carriers created by photons in the vicinity of the active area of one photodetector are not collected by an adjacent photodetector. Photoexcited charge carriers are known to "random walk" while diffusing laterally through a layer of semiconductor material until they are collected by an active detector region. This results in a charge carrier being collected by an active detector region which may be located a significant distance away from the point of origin of the charge carrier. The end result is a form of signal noise resulting from optical crosstalk between the detectors.
The above problem is of particular concern in a detector array in which the detector active areas are reduced in size in order to maximize detector impedance and to minimize detector capacitance. In this case relatively large, open "inactive" areas may exist in which photoexcited carriers can diffuse before being collected by the depletion layer of an active detector.
The diffusion process can be described as taking the charge carrier on a random walk between its point of origin and an arbitrary final location. The net diffusion length is a function of the semiconductor material temperature, charge carrier lifetime and charge carrier mobility. An isolated active detector region typically collects charge carriers which have been created by photoabsorption up to approximately a diffusion length away. Depending upon the thickness of the detector material in relation to the diffusion length and other factors, the diffusion process may be considered to occur in two or in three dimensions.
Several conventional approaches to controlling optical crosstalk operate by reducing the probability of charge carrier diffusion to a nearest active detector. These conventional approaches include the provision of groundplanes or guard diodes which themselves collect the charge carriers. Unfortunately, such techniques destroy a percentage of the charge carriers in the process of preventing optical crosstalk. If the charge carriers are destroyed the nearest active detector is unable to collect them as a signal current, thereby decreasing the efficiency of the detector.
The following references discuss aspects of reducing photodetector active area and/or reducing photodetector junction capacitance: U.S. Pat. No. 4,717,946, Jan. 5, 1988 to Godfrey entitled "Thin Line Junction Photodiode", U.S. Pat. No. 4,652,899, Mar. 4, 1987 to Hoeberechts entitled "Radiation-Sensitive Semiconductor Device Having Reduced Capacitance", U.S. Pat. No. 3,794,891, Feb. 26, 1974 to Takamiya entitled "High Speed Response Phototransistor and Method of Making the Same" and Japan Appl. No. 56-113769, June 9, 1982 entitled "Photo-Receiving Semiconductor".
Commonly assigned in U.S. Pat. No. 4,639,756, Jan. 27, 1987 to Rosbeck et al. entitled "Graded Gap Inversion Layer Photodiode Array" describes a mesa-type array of photodiodes wherein grooves extend completely through a radiation absorbing layer 16 and into an underlying buffer layer 14 such that photocarriers cannot readily move to neighboring diodes (col. 6, lines 18-30). In U.S. Pat. No. 4,646,120, Feb. 24, 1987, entitled "Photodiode Array" Hacskaylo describes, in relation to the prior art (FIG. 1), grooves 6 and 7 that extend only partially through a radiation absorbing layer 2. However, this arrangement is said to suffer from cross-talk.
UK Patent Application No. 2,100,927A, published Jan. 6, 1983 by A. B. Dean et al. and entitled "Photo Diodes" describes an array of photodiodes fabricated over a p-type Cd.sub.x Hg.sub.1-x Te substrate. The array includes a CdTe layer that overlies the substrate and into which individual photodiodes are formed. A heating process is said to cause a gradual material composition change creating a graded heterostructure where the p-n junction is close to the heterojunction.
However, none of these references teach the advantages made possible by providing an array of radiation detecting devices formed within a radiation absorbing layer having a varying composition and energy bandgap, the individual devices being differentiated one from another with grooves and possibly other structure that extend only partially into the radiation absorbing layer.